Why do mountaintops stay snowy, even though they’re closer to the Sun?

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Why do we see snow on mountain peaks that are closer to the Sun but not near the ground? – Mrs. Drews’ third grade class, Beechview Elementary School, Farmington Hills, Michigan

There’s nothing better than a bluebird day in the mountains – a cool, sunny day with a dusting of fresh snow. But why doesn’t the Sun quickly melt all that high altitude snow?

It all comes down to our atmosphere, and that’s what I research as a scientist in Colorado. Let’s go!

Our atmosphere: the armor of the Earth

The Earth’s atmosphere begins at its surface and extends to outer space. It is filled with a mixture of many different gases. Gases in the atmosphere include the oxygen we breathe and the water vapor that causes rain and snow. They are essential to sustaining life on Earth in several ways.

One of the most important jobs of these gases is to protect us from harmful elements in space, including our closest star: the Sun.

Solar radiation provides heat to our planet, but too much can cause problems. If you’ve ever had a sunburn, this idea is already familiar to you.

Some of our atmospheric gases limit the amount of solar radiation that can reach the Earth’s surface by absorbing some of it, preventing temperatures from getting too high during the day. At night, certain atmospheric gases also trap some of the heat that the Earth’s surface releases as it cools, protecting us from unsurvivable cold.

The way the atmosphere regulates Earth’s temperatures is known as the greenhouse effect. You will often hear this term used alongside climate change or global warming. Indeed, global warming is caused by the increase in the greenhouse effect: as people burn fossil fuels in their cars and factories, the amount of greenhouse gases in the atmosphere increases. These extra gases allow Earth’s atmosphere to trap more heat, causing temperatures to rise.

The atmosphere likes to stay grounded

If we compared the Earth’s atmosphere along a Caribbean beach to that surrounding the summit of Mount Everest, the situation would be very different.

This is because as you go higher in the atmosphere, it becomes “thinner”, meaning there are fewer gases present at higher altitudes.

For what? Blame it on gravity.

In the same way that gravity keeps people and objects from flying into space, Earth’s gravitational force pulls gases in our atmosphere, trying to keep them as close to Earth as possible.

As a result, there are fewer gas molecules in the atmosphere as you go higher in altitude, making the air thinner or less dense. Humans can sometimes suffer from altitude sickness at high altitudes because there is less oxygen present in the air due to this phenomenon.

Closer to the Sun, but still cold and snowy?

Our high altitude mountains protrude into the higher altitudes of the atmosphere, where the air contains fewer gas molecules. Although this thinner air allows more solar radiation to pass compared to the atmosphere at sea level, thinner air tends to be colder for two reasons:

First, collisions between gas molecules generate heat, and if you have fewer molecules available to collide with each other, this heat generation is less.

Second, a thinner atmosphere is less effective at retaining heat because there are fewer molecules available to trap and retain heat.

Colder temperatures can create more opportunities for precipitation in the form of snow rather than rain, which is why some mountains can be so snowy.

And if the ground is usually covered in snow, as is the case in many mountain ranges, it can be even easier to maintain cooler temperatures. This is because snow-covered surfaces are highly reflective, making them very effective at bouncing the Sun’s rays back into space instead of being absorbed by the ground.

So if you’re visiting the mountains for some snow fun, be sure to pack your jacket, but don’t forget your sunscreen either.

This article is republished from The Conversation, an independent, nonprofit news organization that brings you trusted facts and analysis to help you make sense of our complex world. It was written by: Allie Mazurek, Colorado State University

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Allie Mazurek does not work for, consult, own shares in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond her academic appointment.